Sintered silicon carbide

ABSTRACT

A DENSE SILICON CARBIDE PRODUCT IS DESCRIBED. THE PRODUCT HAS A FLEXURAL STRENGTH ABOVE 100,000 P.S.I. AT ROOM TEMPERATURE, ABOVE 80,000 P.S.I. AT 1200*C., ABOVE 60,000 P.S.I. AT 1375*C. AND ABOVE 45,000 P.S.I AT 1500*C. THE PRODUCT HAS A GRAIN SIZE OF LESS THAN 5 MICRONS AND ESENTIALLY ALL OF THE SILICON CARBIDE IS IN THE ALPHA FORM. THE PRODUCT HAS A DENSITY IN EXCESS OF 99% OF THEORETICAL DENSITY AND CONTAINS ABOUT .5% TO 5% ALUMINUM. A PREFERRED PROCESS FOR PREPARING THE PRODUCT IS ALSO DESCRIBED.

United States Patent Office Patented Sept. 17, 1974 3,836,673 SINTEREDSILICON CARBIDE Gerald Q. Weaver, Worcester, and Bradford A. Olson,

Leicester, Mass, assignors to Norton Company, Worcester, Mass. NoDrawing. Filed Mar. 23, 1972, Ser. No. 237,545 Int. Cl. C01b 31/36 US.Cl. 423345 1 Claim ABSTRACT OF THE DISCLOSURE A dense silicon carbideproduct is described. The product has a flexural strength above 100,000p.s.i. at room temperature, above 80,000 p.s.i. at 1200 C., above 60,000p.s.i. at 1375 C. and above 45,000 p.s.i. at 1500 C. The product has agrain size of less than 5 microns and essentially all of the siliconcarbide is in the alpha form. The product has a density in excess of 99%of theoretical density and contains about .5% to 5% aluminum. Apreferred process for preparing the product is also described.

BACKGROUND OF THE INVENTION There has long been a need for hard, strongmaterials which can operate under high stress at elevated temperatureswithout undergoing degradation and oxidation. Such materials preferablyshould also have high fiexural strength at room temperature as well asat high temperature. In the past, some eliorts have been made to producesuch materials by forming products of silicon carbide. One productcomprising resolidified silicon carbide is described in Hall Pat.3,158,442 as being made by hot pressing silicon carbide at a temperatureabove 3500 C. and a pressure above 10,000 p.s.i. The flexural strengthof this material is unknown but it apparently never reached the marketplace. One of the more publicized efforts to produce high strengthsilicon carbide has been the KT silicon carbide produced by theCarborundum Company (see Materials and Methods October 1956, page 92;Journal of Material Science, Vol. 6, 1971, pages 324-331). This materialis selfbonded silicon carbide which has some interesting properties butis not very strong, having a fiexural strength less than 50,000 p.s.i.at room temperature. An improved form of silicon carbide which wasproduced by chemical vapor disposition (CVD) has been described byGulden (Journal of American Ceramic Society, Vol. 52, #11, pages 585-590, November 1969). This latter material, which is generally preparedby thermal decomposition of a silane, such as methyltrichlorosilane, hasinteresting physical properties. Its fiexural strength is comparable tothat produced by the present invention but it has the disadvantage thatit cannot be prepared readily in large shapes. It is extremely difficultto form in the shape of balls, turbine blades and the like.Additionally, the product of the present invention has a randomlyoriented grain structure while the CVD product is highly oriented in onedirection. Another material recently announced by Ceradyne, Inc. is madeby an unknown process and has a flexural strength approaching 50,000p.s.i. A hot pressed material produced by Alfred Ceramic Enterprises isreported to have properties similar to the Ceradyne material.

PRIOR ART The Prior Art most nearly applicable to the present invention(other than the above-mentioned materials) is derived primarily fromsome early work by Alliegro, Coffin and Tinklepaugh (J. Am. Ceram. Soc.39 (11) 386-389, 1956) who produced a fairly dense (98%) hot pressedsilicon carbide having a flexural strength on the order of 54,000 poundsper square inch at room temperature. Some recent work by GeneralElectric Company (as described in the Progress Reports prepared for theDepartment of the Navy under Contract N0001971C- 0290) is a furtherextension of the work of Alliegro et al. and involved the addition ofboron to various silicon carbide powders with hot pressing to provideproducts having physical properties comparable to those of Alliegro etal. A recently issued patent to Du Pont, #3,520,656-, July 14, 1970,Meadows & Yates, describes (example 34) a product formed from 96 volumepercent beta silicon carbide and 4 volume percent alumina. This mixtureis hot pressed to form a product having a density of 98% theoretical,about 50% alpha silicon carbide and about 50% beta silicon carbide withinterposed crystals of alumina. The flexural strength of this materialis 94,000 p.s.i. at room temperature. This Du Pont product has neverbeen commercialized. Another remotely relevant patent is #3,178,807 toBergmann (Du Pont) which described the explosive compaction of a SiCskeleton with impregnation with aluminum.

BRIEF SUMMARY OF INVENTION The present invention is predominantlyconcerned with the production of a very dense, strong body of siliconcarbide which, in a preferred embodiment, is prepared by hot pressing anintimate mixture of fine alpha silicon carbide grain and .5% to 5% offine aluminum powder. This mixture is preferably prepared in a ball milllined with tungsten carbide containing tungsten carbide balls by millingfor a period of at least 15 hours of provide an exceedingly intimatedispersion of the aluminum powder throughout the alpha silicon carbidepowder. The alpha silicon carbide powder preferably has a grain size onthe order of 1-5 microns and is of a purity on the order of at least99%. The mixing, which preferably takes place in an inert medium, suchas isopropanol, should be carried on for at least 15 hours. The strengthof the final pressed product is almost directly proportional to the timeof milling in the range between 1 and 60 hours. The finely milledproduct is then dried by filtration and heating to C. to drive off theremainder of the isopropanol. The product is then hot pressed in agraphite mold (with a minimum exposure to air) to a temperature of atleast 1950 C. and a pressure of at least 2000 p.s.i. maximum pressureand temperature being maintained for at least one minute. The hotpressing is preferably carried out in an atmosphere of argon. Theproduct resulting from this practice of the invention has a density inexcess of 99% of theoretical density.

DETAILED PRACTICE OF INVENTION In order that the invention may be morereadily understood, one preferred method of practicing the invention isset forth in the following nonlimiting example:

EXAMPLE I The hot pressed SiC produced here started with 3 micron alphaSiC powder. The SiC powder used was a commercial Norton powder having anaverage particle size of 4.5 microns (as determined by micromerograph)with a boron content of less than 50 p.p.m., aluminum content of lessthan 0.1% and an iron content of .1%. The aluminum powder used wasReynolds 400 Atomized aluminum powder and is commercially available. Thetwo powders were blended together by ball milling in tungsten carbidemills using tungsten carbide balls. The mill used in this example was aone quart size. The charge size was 200 gms. of 3 micron silicon carbideplus 6 grams of aluminum. The mill was filled to half its volume by WCballs approximately /2" in diameter. To the charge was added 400 ml. ofisopropanol. The material was ball milled for 31 hours at r.p.m. Aftermilling,

the material was unloaded from the mill and placed in an oven for 12hours at 80 C. to remove the isopropanol. The dried powder was thenpassed through 'a 40 mesh screen. This procedure produced a fine, wellblended powder which could now be hot pressed.

strength. The exact role of the aluminum is not completely understoodbut it appears to be almost completely diffused into the crystal latticesince X-ray diffraction techniques fail to disclose the presence ofappreciable quantities of a separate aluminum-bearing phase in the Thedies used in the hot pressing were made of graphite 5 final product.Examination of a fracture surface of the with graphite plungers. Thecavity of the die produced a product by a scanning electron microscopeat a mag- '3.5" diameter x /2" thick shape. The inside of the die,nification at 25,000 times and by electron probe analysis as well as thecontact surfaces (inside ends) of the plungof a magnification of 5000times failed to'detect any ers,were coated with boron nitride. phaseother than SiC and WC. There appears to be no The hot pressing operationis carried out in an inducappreciable concentration of aluminum oxide,aluminum tion heated furnace (graphite) using argon gas as a purge.carbide, or aluminum silicide as a separate phase. While The furnace ismounted on a 75-ton hydraulic press. As some trace indications ofaluminum compounds do exist, the temperature is increased from roomtemperature to there are nowhere near enough to account for the amountapproximately 1400 C., the pressure is increased from 0f the aluminumadded to the product and still existing 100 p.s.i. to approximately 2700p.s.i. From 1400 C. to (eg. 2%) in the product as determined by chemicalor 2075 C. the pressure is maintained at a constant 2700 spectrographicanalysis.

p.s.i. The furnace is held at 2075 C. for 2 minutes. The While thetungsten carbide may be beneficial, it is not furnace is allowed to coolto room temperature while the believed to be essential since the volumepercent of WC die remains under full pressure of 2700 p.s.i. varies,over a substantial range and has only a minor The resultant piece had adensity of 3.627 gm./cc. Test effect On Strength, as Seen in Table bars4; X A; x 3 inch) were machined from the piece In the above discussionof the invention, a specific emand tested on an Instron test rig(.O2"/minute cross h d bodiment has been detailed involving the use ofaluminum speed). Room temperature cross bending values on /s pe While iis Preferred, ether aluminous mate" inch x /s inch cross-sectionaveraged 130,270 p.s.i. with W115 Such as aluminum halides, Which r l ssStable h a standard deviation of 11,186 p.s.i. for a .75 inch span.aluminum Oxide, can be employed- The Prlnelpal requlre' As mentionedabove, the fle r t en th for. the rodments for these other aluminousmaterials are that they net of the present invention is directly relatedto the millare reducible to [he y fine Particle i durlng the 1 ing timed in hi h th lu i powder d ili ing to provide an extremely intimatemrxture with the S111- carbide are mixed in the WC'mill. The density ofthe prod- Con Carbide and that y be suffielenfly unstable P the net isalso directly related to milling time but the inhot pressing temperatureZll'lLlPI S ICjSO that atomlc alucrease indensity is primarily due toincrease in the WC minum is provided for diffusion into the siliconcarbide content of the product. crystal lattice In Table I below thereare tabulated the results from While an upper'limit of approximately 60hours has a number of different runs which were identical to the 3? beenmentioned above in connection with the milling procedure of Example I,except for the milling time 0 time, this is not critical. There areseveral factors which and size of mill. The small mill was one quart andthe should be considered in determiningthe time for stopping large millwas one gallon. In this table, the flexural the milling. One isobviously process cost; but more imstrengths are averages over a numberof runs. portant is the particle size of the aluminum powder. WhileTABLE 1 Flexural strength (0005 p.s.i.) Density C. Volume Percent.percent Time, hours 20 1, 200 1,375 1,500 G./cc. other Mill 84 56 0Small 90 64 .25 Large 90 63 1.1 Small 106 78 2.3 Do. 110 B2 3.5 Do. 13088 3.6 Do. 117 75 .95 Large In the above discussion of the invention,the preferred the smallest possible particle size is desirable from theembodiment contemplates the use of fine aluminum powstandpoint ofuniformity of dispersion with the SiC powder which is milled with thesilicon carbide powder for a der. there is the practical problem of theenormously inconsiderable period of time, preferably in excess of 15creased reactivity of the powder as a function of the rehours. Aspointed out above, the strength of the hot duction in particle size.Accordingly, the milling must be pressed product increases directly withthe milling time. stopped before the aluminum powder is reduced in sizeto It is believed that this is the result of the continual the pointwhere it becomes reactive with the inert isoreduction in size of thealuminum particles during the propanol. wet milling. It is believedthese aluminum particles are Similarly the aluminum powder should not beso small reduced to a size of less than 0.1 micron as a result of thatthe surface oxide, which will inevitably form during the milling andthat most of the aluminum particles are subsequent processing, willconvert a substantial portion smeared onto the surface of the siliconcarbide. Even if of the powder to the oxide. these aluminum particlesare examined under a scanning In the specification and claims, theexpressions perelectron microscope at a magnification of 25,000 timescent and mean weight percent, except where otherafter only five hours ofmilling, the aluminum powder wise indicated. apparently has been smearedsufiiciently so that indi- While the invention has been described abovein convidual aluminum particles cannot be identified. nection with thepreferred example wherein aluminum When the mixture of the siliconcarbide and very fine powder is added to silicon carbide powder andmilled in aluminum is heated under pressure to a temperature on atungsten carbide lined mill, the same result can be the order of 2000C., it is believed that the aluminum achieved by adding silicon carbidepowder alone to an enters into the silicon carbide crystal lattice by adiffusion aluminum lined mill containing aluminum balls or rods.mechanism. As a result of this diffusion into the surface In this lattercase, the abrasive silicon carbide grains reof the crystal, there is achange in the surface energy of move enough aluminum from the mill androds to incorthe SiC crystal with resultant promotion of crystal toporate between .5% and 5% of aluminum dispersed crystal bond wh1ch givesthe product 1ts remarkably high throughout the milled product. On hotpressing, this milled product will provide the Same type of dense, highstrength product described previously.

What is claimed is:

1. A dense, homogeneous, hot pressed silicon carbide body having aflexural strength above 100,000 p.s.i. to about an average of 130,270p.s.i. with a standard deviation of 11,186 p.s.i. for a .75 inch span atroom temperature, said body while having an average grain size betweenabout 1 and 5 microns, the silicon carbide consisting essentially ofalpha silicon carbide, the product con- 10 taining between about .5 and5% aluminum uniformly dispersed throughout the silicon carbide,essentially all of said aluminum being undetectable as a separate phaseby X-ray detection techniques, said body being substantially free of anysecond phase (other than WC) which is detectable by a scanning electronmicroscope image of a fracture surface at a magnification of 25,000times or an electron microprobe analysis of a fracture surface at amagnification of 5000 times.

References Cited UNITED STATES PATENTS 3,532,493 10/1970 Chay 752043,249,407 5/1966 Alexander et a1. 75204 3,178,807 4/1965 Bergmann 752033,521,825 7/1970 Morcom 2412t5 3,520,656 7/ 1970 Meadows et al 423-34.!

OTHER REFERENCES Alliegro et al.: Pressure Sintered Sic, Journal of theAmerican Ceramic Society, Vol. 39, No. 11, pp. 386- 389, November 1956.

OSCAR R. VERTIZ, Primary Examiner B. E. HEARN, Assistant Examiner US.Cl. X.R. 29182.7

